The conversion of synthesis gas--a mixture of hydrogen, carbon monoxide and other gases such as carbon dioxide and methane--into hydrocarbons has been of interest for more than fifty years. This conversion is frequently referred to as the Fischer-Tropsch (F-T) synthesis in honor of the pioneering work of Franz Fischer and Hans Tropsch in the early 1920's (C. Satterfield, Heterogeneous Catalysis in Industrial Practice, p. 432-442 (2d Ed. 1991)).
All large-scale F-T reactors built to date have utilized either fixed-bed or fluidized-bed reactors (See, e.g., G. Stiegel, PETC Review, p. 14-23 (Fall 1991); Fischer-Tropsch Synthesis--The SASOL high-efficiency synfuels process, Sasol Technology (Pty) Ltd., South Africa). However, a good deal of research and development in the United States, the United Kingdom, and Germany has been devoted to a different reactor concept known as the slurry bubble column (SBC) reactor. (See, e.g., H. Kolbel and M. Ralek, Catal. Rev.-Sci. Eng., 21, 225 (1980)).
Slurry bubble column reactors have distinct advantages over fixed and fluidized bed reactors. Some of these advantages are generic, e.g., a very close approach to isothermal operation, simple construction leading to low capital cost, and the ability to continuously withdraw and add catalyst in order to maintain a constant level of catalyst activity in the reactor. A major advantage that is specific to F-T chemistry, and related reactions such as alcohol synthesis, is the ability of SBC reactors to operate with a feed gas that contains a high ratio of carbon monoxide to hydrogen. Such CO/H.sub.2 ratios are characteristic of modern, thermally-efficient coal gasifiers. The use of a SBC reactor in conjunction with a thermally-efficient coal gasifier can lead to a significant reduction in the cost of producing hydrocarbon liquids from coal, and in the overall thermal efficiency of the coal-to-liquids process, relative to current commercial technology.
An important and difficult problem which must be overcome before SBC reactors can be widely used is separating the small catalyst particles in the viscous slurry from the liquid hydrocarbon product. This separation is essential to the commercial implementation of F-T technology based on slurry bubble column reactors for several reasons. First, the liquid product from F-T synthesis must undergo further processing in catalytic reactors. The presence of catalyst particles in the liquid interferes with subsequent processing steps such as hydrocracking and distillation. Second, the portion of the catalyst slurry that is withdrawn for either regeneration or disposal should be as concentrated as possible in order to minimize the amount of valuable hydrocarbon liquid that must be processed during catalyst regeneration or disposal.
P. Zhou, Status Review of Fischer-Tropsch Slurry Reactor Catalyst/Wax Separation Techniques, Burns and Roe Service Corp. (February 1991) recently reviewed previous work on F-T slurry catalyst/wax separation, including techniques such as vacuum distillation, thermal cracking of vacuum bottoms, sedimentation, filtration, various forms of centrifugation, high-gradient magnetic separation (HGMS), solvent-assisted catalyst/wax separation and chemical methods. The review concludes that no single technology is entirely satisfactory, and recommends certain "hybrid" approaches. Accordingly, there is a continued need for new methods and apparatus for the separation SBC reactor slurry into F-T catalyst and the hydrocarbon product.